This invention generally relates to magnetically levitated vehicles; and more specifically, to reducing the magnetic field strength in the passenger compartments of magnetically levitated vehicles.
In a magnetically levitated vehicle, a group of magnets located on the vehicle are used to hold the vehicle above a guideway. This is accomplished by propelling the magnets a few inches over the guideway, which is installed with aluminum coils or aluminum sheets along the full length of the guideway. The vehicle magnets induce magnetic fields in those aluminum coils or sheets; and these induced magnetic fields repel the vehicle magnets, causing the vehicle to levitate above the guideway. The higher the speed of the vehicle over the guideway, the greater the repulsive or levitation force between the vehicle magnets and the magnetic fields induced in the aluminum coils or sheets on the guideway.
Recently, attention has been directed to using superconducting magnets--that is, electromagnets in which an electric current is conducted through a superconducting material to produce a magnetic field--on magnetically levitated vehicles. Such magnets are capable of producing very strong steady state magnetic fields and alternating magnetic fields, referred to as dc and ac fields, respectively; and for example, magnetic field strengths of 10,000 to 20,000 gauss may be obtained with such magnets. When such magnetic fields are employed to levitate vehicles, however, the strengths of the magnetic fields in the passenger compartments of the vehicles may reach strengths, such as 100 to 200 gauss, that may be undesirably high. In particular, the magnetic fields in the passenger compartments may interfere with the operation of electrical equipment, and may have other undesirable effects.
Heretofore, two general procedures have been used to reduce or to minimize the strengths of the levitating magnetic fields in the passenger compartments of magnetically levitated vehicles. With a first of these procedures, passive devices such as soft iron or mu-metal sheets are used to shield the passenger compartments from the levitating magnetic fields. With the second of these procedures, active coils are used to null the effects of the levitating fields in the passenger compartments. The use of passive devices is generally undesirable because these devices are normally quite heavy and only shield the passenger compartments from the dc components of the magnetic fields. Active devices, which generally comprise superconducting coils, are significantly lighter than passive devices and can be arranged in a wide variety of sizes and geometries to null magnetic fields. However, active nulling coil devices also tend to reduce the effectiveness with which the levitating magnetic fields support the vehicles; and in order to null the ac fields associated with a levitating magnetic field, active devices must be wound in series with the main superconducting coils.